1. Field of the Invention
This invention relates to a high-solids coating composition which is cured by light irradiation and heat, and which is capable of inhibiting the sag of paint without affecting performances of coating film, and also to a process to form a cured coating film with use of said coating composition.
2. Description of Prior Arts
The reduction of the amount of organic solvent used has been one of important objectives in the field of coating composition, from the viewpoint of environmental protection against air pollution and the saving of resources. As a means to achieve this objective, there has been developed so-called xe2x80x9chigh-solids coating compositionxe2x80x9d, i.e., an organic solvent type paint wherein the amount of organic solvent contained in paint is reduced by which to increase solid content.
Simple reduction of the amount of organic solvent contained in an organic solvent type paint causes increase of paint viscosity. If a high-viscous paint which contains a small amount of organic solvent is used for spray coating which is usually employed in such a field as automobile coating, insufficiently atomized paint is applied onto a substrate, and, thus, resultant coating film decreases in smoothness. It is therefore necessary to lower the viscosity of a high-solids coating composition to such an extent as to be suitable for spray coating. For this purpose, there have been proposed an idea of lowering the molecular weight of resin components in paint and an idea of decreasing the polarity of resin.
Thus proposed coating compositions are atomizable in spray coating. However, only a small amount of solvent is volatilized during the time from when the coating composition is thrown from spray nozzle up to the time when it is applied onto substrate. On this account, the increase of viscosity of the applied paint is small, and, thus, the viscosity of applied paint is lower than conventional organic solvent type paint, with the result that the applied paint is liable to sag. In particular, when substrate is placed in a heat-drying oven in a baking step, the temperature of applied paint increases and causes the decrease of viscosity, resulting in sagging, before thermosetting reaction occurs.
With a view to preventing the sag of paint, it has been known to add a fluidity controlling agent to paint. In this case, however, fluidity controlling agent usually needs to be blended in larger amount than in conventional organic solvent type paint, in order that fluidity controlling agent may produce enough effects in the above-mentioned high-solids coating composition. The blending of a large amount of fluidity controlling agent has, however, caused problems that the appearance of finished coating film such as gloss is marred, that a coating film composed of two or more coats superposed decreases in inter-layer adhesivity, or that the water resistance of a coating film lowers. When, on the other hand, resin to be contained in a coating composition is rendered low-molecular or low-polar by which to attain high solids content, there are caused grave defects that coating film performances such as coating film hardness, acid rain resistance and scuff resistance are spoiled.
Japanese Patent Application Laid-Open (Kokai) No. 70471/95 discloses a method of applying, by spray-coating, a high-solids paint which comprises a macromonomer having ethylenically double bond at one end and a photopolymerization initiator, wherein sag is prevented by irradiating flying paint particles with ultraviolet ray. In this method, the increase of viscosity is achieved by the radical polymerization of macromonomer, and no three-dimensional crosslinkage occurs, and, therefore, smooth coating film can be obtained. However, macromonomer needs to be used in such a large amount as usually 30% by weight or more on the basis of solid content of paint, for the purpose of the prevention of sag. This has caused problems that the solid content of paint cannot be increased, or that coating film performances such as weatherability are liable to be spoiled.
International PCT Publication WO 97/20642 discloses a method of a coating film formation which comprises, in spray-coating of a paint which contains epoxy group-containing resin and photocationically polymerization initiator, irradiating the paint thrown from spray gun with active energy ray and then heat-curing thus applied coating film. In this method, however, there have been problems that the amount of basic components to be blended needs to be regulated to such an extent as not to inhibit cation polymerization, that a coating film which is finally obtained after baking has sometimes poor performances, or that cationic polymerization initiator causes a coating film to yellow.
The objective of this invention is to provide a coating composition which is free from the above-mentioned defects of conventional techniques, and which has low viscosity in spite of high solids content, which causes no sagging of coating film in coating steps and gives good appearance of finished coating film, and which is capable of forming a cured coating film excellent in coating film performances such as coating film hardness, acid rain resistance and scuff resistance.
Another objective of this invention is to provide a method of forming, with use of the above-mentioned coating composition, a cured coating film which is excellent in coating film performances such as coating film hardness, acid rain resistance and scuff resistance, said method causing no sagging in coating steps and gives good appearance of finished coating film.
Other objectives and characteristic features of this invention will be clearly seen in the following.
Assiduously studying with a view to achieving the above-mentioned objectives, inventors of this invention have developed a new coating composition which comprises radically polymerizable initiator, specific hydroxyl group-containing compound, curing agent and photopolymerization initiator, and found out that, by means of irradiating said coating composition with light, either during spray coating or immediately after applied, there can be formed a cured coating film which satisfies the above-mentioned requirements, and, thus, have completed this invention.
This invention provides a high-solids coating composition which comprises:
(A) a compound which has at least one radically polymerizable unsaturated group per molecule,
(B) a hydroxyl group-containing ester compound which is obtained by ester-forming addition reaction between 2,2-dimethylolalkanoic acid of the following formula (I): 
wherein R2 is an alkyl group having 2 to 4 carbon atoms, and alkanoic acid monoglycidylester of the following formula (II): 
wherein R1 is an alkyl group having 4 to 20 carbon atoms, and which has an acid value of 20 mgKOH/g or less,
(C) a curing agent which is reactive with the above-mentioned hydroxyl group-containing ester compound (B), and
(D) a photopolymerization initiator.
This invention also provides a method for forming a cured coating film by spray-coating of the above-mentioned coating composition, which method is characterized in that flying paint particles which have been sprayed and/or paint which has been applied onto substrate are irradiated with active energy ray by which to cause partial reaction of the coating composition, and subsequently heating thus applied coating film.
This invention further provides a method of forming a multi-layered coating film which comprises at least one layer of colored coating film and at least one layer of clear coating film, the uppermost layer being a clear coating film, which method is characterized in that the uppermost clear coating film is formed by the above-mentioned method for forming a cured coating film.
In the following, the coating composition of this invention and a method for forming a cured coating film with use of the coating composition are explained in more detail.